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Multilayer PCB Design: HDI Stackup Demystified

stack up

Whether you believe Moore’s Law is dead or alive, the strong economic incentive to pack more processing power into ever smaller form factors is unlikely to abate any time soon. While experts predict we’ll reach the physical limitations of transistors some time in the 2020s, the truth is there is still a lot a PCB designer can do at the macro board level to fit more components onto smaller boards.  

Enter the HDI stackup—a technology at the cutting edge of multilayer PCB design that promises to help PCB designers make smaller complex boards for years to come. Ready to tackle your first multilayer HDI PCB design? In this post we’ll cover the basics behind the HDI stackup.

What is an HDI stackup?

HDI is short for high density interconnect, and refers to the use of buried, blind and micro vias as well as any layer HDIs to create compact boards. Besides the obvious benefit in packing the same board functionality into a smaller footprint, HDI comes with a number of benefits when compared to traditional multilayer designs:

  • Fewer layers

  • Improved signal integrity

  • Lower power consumption

  • Better electrical performance

At the heart of these benefits is the fact that reducing signal path throughout your board will naturally improve signal integrity and electrical performance, provided that you are able to properly account for EMI/EMC considerations.

Let’s take a closer look at the features that make HDI possible:

  • Microvias are extremely small vias (often laser drilled) with an aspect ratio (depth to diameter) of 1:1

  • Blind vias connect an exterior layer to at least one interior layer without penetrating the entire board.

  • Buried vias connect one or more inner layers together without any connections to the exterior layers of a board.

  • Any layer HDI (or ELIC) refers to the use of stacked copper-filled microvias to connect multiple layers in a PCB.

Types of HDI Stackups

With the advent of smartphones, the number of  layers a PCB designer has to contend with have increased, even when using the space-saving advantages of HDI technology. HDI stackups are classified using the following shorthand: i+N+i where a 1+N+1 stackup would indicate one sequential lamination on each side of a core. A 2+N+2 stackup would indicate two sequential laminations, and so on for “i” laminations.

PCB Design Considerations

HDI PCB design is like a multi-dimensional puzzle. Here are some common design considerations you’ll need to factor into your HDI PCB design:

  • Impedance Control: You’ll want to maintain tight tolerances (within ±10%) on dielectric layer thicknesses, trace widths, and spacings to ensure impedance does not impact signal integrity.

  • EMI/EMC: All those radiation considerations such as avoiding accidental antennas and noise apply, especially because HDI is used for high speed signal designs.

  • Thermal: HDI often lead to improved thermal performance, however you’ll still want to factor in thermal stability of microvias and trace widths in high-speed signal designs.

The need to factor physical, electromagnetic, and thermal considerations into an HDI PCB design adds a lot of complexity to the design process. Fortunately, EDA (electronic design automation) software tools have evolved to make solving multi-dimensional PCB problems easier. Check out Cadence’s suite of PCB design and analysis tools today.